1. Field of the Invention This invention relates to droplet deposition methods and apparatus in which droplets are ejected from a chamber via a nozzle.
2. Related Technology
In a known apparatus (see for example EP-A-0 277 703 and EP-A-0 278 590) an elongate ink chamber has one or more of the longitudinally extending walls formed of piezoelectric material. By the application of an electric field in a direction appropriate to the poling of that piezoelectric material, the wall can be caused to move into and out of the ink chamber to establish longitudinal acoustic waves in the ink. With appropriate timing of the actuating waveform and with appropriate acoustic reflection at the ends of the chamber, one or a controlled succession of droplets can be ejected through the nozzle.
The nozzle may be situated at one end of the elongate ink chamber in the so-called “end shooter” arrangement or towards the middle of the chamber in the “side shooter” arrangement.
In a printer or other droplet deposition apparatus, care is obviously taken to avoid contamination with (or to remove from the ink) debris or bubbles, which might cause blockage of a nozzle. The occurrence of debris or bubbles cannot however be avoided completely; some debris may be generated through manufacturing irregularities within the printhead and some bubbles may unavoidably form within the printhead as a direct result of the fluid pressure changes that accompany droplet ejection.
To address this problem, it has been suggested to provide in both side shooter and end shooter configurations, a continuous flow of ink past the nozzle in an attempt to sweep away from the nozzle any debris or bubbles which would otherwise cause blockage at the nozzle. This continuous flow occurs whilst the printer is printing and whilst the printer is not printing so that the continuous flow is preferably greater than, and it has been suggested up to ten times greater than, the maximum flow rate through the nozzle.
The continuous or persistent ink flow through the channel can provide significant improvements to the uniformity and reliability of operation. Prior to printing the flow can be used to purge any debris or air from the nozzle, channel, ink manifolds or ink supply system and where necessary the system can include thermal control. Prior to printing it is often necessary to have the system reach thermal stability. During printing and depending upon the pattern to be formed different parts of the actuator are likely to operate at different duties which without constant flow are known to lead to differing operating temperatures increasing the risk of both minor and catastrophic image defects.
The side-shooter with constant recirculation is known to reduce the impact of certain defects by either reducing the time the channel and nozzle are exposed or providing a self-priming mechanism. Some of these are listed below:
Cause of FailureEffectVibrationDisplacement or breakage of nozzle meniscusDebris (dirt)Causes local viscosity distortion can disrupt flowCan inhibit fluid ejection at the nozzleDebris (air)Large air bubbles starve nozzle/channel of fluidSmall air bubbles reduce acoustic efficiency(increase compliance)Bubbles within the channel will grow due to rectifieddiffusionIngested airAccumulated air ingested at the nozzleViscosityChanges to the viscosity of the fluid at a macroscale, due to mild flocculation or contamination, forexample.
The provision of a continuous flow past the nozzle is relatively straightforward in the side shooter configuration. Reference is directed in this regard to EP-A-1 140 513. In this prior proposal, both ends of the ink chamber remain open, simplifying the provision of a relatively high flow rate continuously past the nozzle. This flow across the nozzle is orthogonal to the direction along which droplets are ejected and is thus particularly effective in sweeping debris and bubbles away from the nozzle.
The provision of a continuous flow through the ink chamber is not straightforward in an end shooter configuration. In a prior proposal (see for example U.S. Pat. No. 6,705,704) a barrier divides the ink chamber longitudinally. In use, a continuous ink flow is established in a U-shaped path in the chamber: towards the nozzle on one side of the barrier, across the nozzle, and away from the nozzle on the other side of the barrier. This arrangement has advantages but is not appropriate in all circumstances.